Page 46 - Summer 2021

MOBILE EARTHQUAKE RECORDING
satellite in near real time. That all-important autonomy enabled a series of science deployments. Signal-processing algorithms for sound decision making again proved to be critically important. Detection and discrimination routines became probabilistic (Sukhovich et al., 2011) such that MERMAID now has an evaluation and scoring system that confidently picks out segments of seismological interest and reports them to the receiving data centers.
In this context, one often wonders whether MERMAID technology could be useful for tsunami warning. The ascent from MERMAID’s current parking depth of 1,500 m takes a few hours. Such a delay is immaterial for global earthquake seismology, but the instrument would need to wait even longer before surfacing until a tsunami can be confidently detected, which it would need to do at even lower frequencies than is now typical for earthquake recording (Joubert et al., 2015). Alternatively, MERMAID could be programmed to dive at a shallower depth or reen- gineered to rise to the surface faster. However, because tsunami waves caused by an earthquake in, for example, Chile, reach Tahiti, Hawaii, and Japan about 10, 15, and 22 hours, respectively, after the main event, Masayuki Obayashi from the Japan Agency for Marine-Earth Science and Technology (Yokosuka) calculates that there may well be an opportunity for a worldwide array of MERMAID floats to fulfill this important societal function.
Across the Mediterranean Sea, the Indian Ocean, and the Pacific, second-generation MERMAID established itself as a reliable purveyor of signals from earthquakes large and small. Although noise environments vary vastly among oceans and with the seasons, a large fraction of global earthquakes with magnitudes greater than 6.5 can be recorded, as can many smaller ones (Sukhovich et al., 2015). For example, over the course of one month in 2013, one of the floats unexpectedly reported hundreds of small earthquakes following a magnitude 5.1 shock in the Indian Ocean. Even the closest island stations recorded only a handful of the largest ones, all within a brief interval after the main shock. Hence, although mantle seismologists continue to focus on large and dis- tant earthquakes, scientists interested in the oceanic crust will find MERMAID perfectly capable of being optimized for the study of smaller earthquakes near the instrument.
MERMAID’s first coordinated scientific experiment was dedicated to imaging the mantle roots of the Galápagos
volcanic hot spot. For two years, nine floats sampled seismic ray paths that illuminated mantle corridors that had never before been accessed directly. Tomographic modeling of the new data combined with data from land stations revealed that the Galápagos archipelago is under- lain by a deep-seated mantle plume, with rocks buoyed up by excess temperature carrying an unexpectedly large heat flux toward the surface (Nolet et al., 2019). Those are the types of findings that MERMAID was designed to enable, that is, provocative new observations from previ- ously inaccessible areas, leading to models that stimulate further thinking by the geophysical community.
In making the 10-year leap from prototype to scientific workhorse, the low-cost and nimble MERMAID instru- ment became a vital partner in the seismic exploration of the Earth’s deep interior. Freely drifting midwater hydro- phones fit in an Earth-observing strategy that must also contain increased ocean-bottom sensor coverage among permanent networks of (is)land-based sensors and possi- bly even more exotic types of data gathering (e.g., Sladen et al., 2019). Autonomous hydrophones will not replace ocean-bottom seismometers as the backbone of an ocean- wide observing system, at least in the foreseeable future. But although global arrays of three-component ocean- bottom sensors remain dreams of the future, MERMAID rules in the ocean today.
Third Is the Charm
Yann Hello at Géoazur in Valbonne, France, and a team at French engineering firm OSEAN SAS in Le Pradet, France, solved the last of the sticking points, longevity. Although data returns are variable depending on seis- micity, MERMAID’s life expectancy is now five years. Moreover, at every surfacing, MERMAID not only transmits seismograms but also takes instructions, for example, to update mission parameters or tweak filter settings. A glass sphere now encapsulates the batteries, electronics, and hydraulic components to achieve neutral buoyancy in the water column (Figure 5). On its first day-long deployment out of Kobe, Japan, a third-gen- eration MERMAID was again so lucky as to catch a first earthquake, which arrived in the form of a local mag- nitude 5.2 event, dutifully reported over the IRIDIUM satellite network.
In the current commercial version, MERMAID can maintain acoustic operability down to about 3,000 m.
46 Acoustics Today • Summer 2021